1. Field of the Invention
The present invention relates, in general, to a safety injection tank used for quickly injecting emergency core cooling water (ECCW) to a reactor vessel in the case of a cold leg large break accident (CLLBA) in a pressurized water reactor (PWR) and, more particularly, to a technique related to a fluidic device configured to efficiently execute the transition of ECCW injection mode from a high flow injection mode in an early stage of the CLLBA to a low flow injection mode in a latter stage of the CLLBA. Particularly, the present invention relates to a technique of executing the transition of ECCW injection mode from a high flow injection mode to a low flow injection mode using a gravity-driven fluidic device, installed in the safety injection tank, as time goes by after operation of the safety injection tank.
2. Description of the Related Art
Pressurized water reactors (PWR) must be designed, constructed and operated according to rigid safety standards and, particularly, an emergency core cooling water injection system (ECCWIS), provided against a large break loss-of-coolant accident (LBLOCA) caused by a cold leg break accident (CLBA), is estimated as an important device in a reactor cooling system.
The present invention relates to a safety injection tank, which is a vessel constituting the emergency core cooling water injection system (ECCWIS) and is used for storing the emergency core cooling water (ECCW) therein. Nitrogen gas is charged in an empty upper space in the safety injection tank, so that the safety injection tank can passively inject the ECCW to a reactor system due to a pressure difference.
When a large break loss-of-coolant accident (LBLOCA), in which the safety injection tank must be operated, occurs in a reactor system, it is necessary for the safety injection tank to inject a high flow of ECCW into the reactor system in an early stage of the LBLOCA and to inject a low flow of ECCW in the latter stage of the LBLOCA. In the early stage of the LBLOCA, in which the reactor core is exposed, the ECCW must be quickly injected by the discharge of the high flow of ECCW from the tank, but in the latter stage of the LBLOCA, in which a substantial amount of ECCW has been charged in the reactor system and reaches a predetermined water level, it is required for the safety injection tank to discharge only a low flow of ECCW so as to compensate for lost ECCW, which has been lost to the outside of the reactor core.
A conventional vortex type fluidic device, used in the safety injection tank, uses a method of flow mode transition from the high flow injection mode to the low flow injection mode using a height difference between the height of an inlet port provided in the upper end of a vertical pipe and a water level in the safety injection tank.
FIG. 1 shows a safety injection tank disclosed in Korean Patent No. 369247. As shown in (a) of FIG. 1, during a high flow injection mode, in which the water level in the safety injection tank 10 is higher than the height of a supply line inlet port 20, the emergency core cooling water flows into both the supply line inlet port 20 and a control line inlet port 30, passes through a supply line 21 and a control line 31 and meets together at portions around the inner circumference of a vortex chamber 55, and flows inwards in radial directions in the vortex chamber 55 prior to being discharged from the vortex chamber 55 through a drain port. Meanwhile, as shown in (b) of FIG. 1, during a low flow injection mode, in which the water level in the safety injection tank 10 is lower than the height of the supply line inlet port 20, the emergency core cooling water cannot flow into the supply line inlet port 20, but the emergency core cooling water flows into only the control line inlet port 30 and is discharged from the vortex chamber 55 through the drain port while forming a strong vortex in the chamber 55.
However, the fluidic device disclosed in the Korean Patent No. 369247 is problematic in that the device has a complicated inner structure, as shown in the cross-sectional views of (a) and (b) of FIG. 1, and the characteristics thereof may be easily changed according to a change in the swirling direction of the vortex, a surface area ratio and a relative angle between respective discharge lines, so that it is very difficult to estimate the characteristics of a turndown ratio using the surface area ratio between the high flow inlet port and the low flow inlet port. To design a stable fluidic device capable of efficiently responding to desired characteristics of a reactor system, it should be required to estimate a turndown ratio between the maximum flow rate and the minimum flow rate according to a simple flow area ratio. However, the fluidic device shown in FIG. 1 cannot efficiently estimate the turndown ratio between the maximum and minimum flow rates.
Another problem of the fluidic device shown in FIG. 1 resides in that the large flow supply line inlet port 20 is exposed to the nitrogen gas in the safety injection tank 10 during a low flow injection mode, as shown in FIG. 2 schematically illustrating both a turndown point and a time to early inject the nitrogen gas when the injected flow rate is changed from a high flow rate to a low flow rate, so that an early injection of nitrogen gas, in which the nitrogen gas is early injected into the reactor system along with the emergency core cooling water, may occur during the low flow injection mode. When the nitrogen gas is early injected into the reactor system as described above, the nitrogen gas may disturb condensation of steam in the reactor system and reduces the precision of thermal hydraulic analysis of the reactor system, thus causing some problems in the reactor system.
In addition to Korean Patent No. 369247 shown in FIG. 1 of the accompanying drawings, another example of conventional safety injection tank may be referred to Korean Patent No. 402750 and Japanese Patent Application laid-open Publication No. Hei 4-328494. However, each of the above-mentioned prior art safety injection tanks has a structure, which cannot avoid the early injection of nitrogen gas.
In an effort to prevent the early injection of nitrogen gas, a technique of closing the inlet port provided in the uppermost end of a vertical pipe 20 of the fluidic device using a buoyant plate 50 is disclosed in Korean Patent No. 556288 (see FIG. 3). Typically, materials with low specific weight have been used as the material, which can float in a boric acid solution of high boric acid content charged in the safety injection tank. However, the buoyant plate made of a material with low specific weight is almost impossible to cope with a desired life soundness, which requires the buoyant plate to endure without damage for 40-60 years that are the typical life spans of nuclear power plates. Further, when the fluidic device is continuously kept in a standby state without being operated for a lengthy period of time, the buoyant plate may be fixed to the inlet port of the vertical pipe and fails to start its operation, so that it is very difficult to use the buoyant plate in a practical safety injection tank.